Human Microscopic Anatomy INDEX Laboratory #8
CHA 402/202

Integument

Skin from various areas of the body varies in thickness, in presence or absence of hairs or apocrine sweat glands, and in relative concentration of eccrine sweat glands. Skin consists of two main parts: l) the epidermis, a stratified squamous epithelium; and 2) the dermis, the connective tissue layer beneath the epidermis.

a-41, (782) axilla and foot. This slide has both thick and thin skin. In the thick skin (plantar surface of the foot), note the irregular junction of epidermis with underlying dermis. The deepest portions of the epidermis are called epidermal ridges or rete ridges (see Micrograph #1 right). The basal layer of epidermis is the columnar stratum basale or stratum germinativum, since this is the site of most mitotic activity. The cells above this layer are polyhedral, and because of their many desmosomes appear spiny, hence the layer is referred to as the stratum spinosum. The layer of cells which contain granules is the stratum granulosum. Above that layer are the multiple layers of dead cells that make up the stratum corneum. See EM DEMO 1 .

Ultrastructural aspects of differentiation in the skin can be seen in EM DEMO 2 . As the keratinocytes move upward and differentiate, keratin filaments (a type of intermediate filament) aggregate to form larger groups of filaments (tonofilaments) and even larger tonofibrils which are visible by light microscopy in the spinosum layer. During differentiation, in the stratum granulosum, aggregations of keratin filaments become embedded in matrix proteins derived originally from keratohyaline granules. In the cells of the stratum corneum, this keratin protein complex occupies the entire dead cell.

The dermis has a number of vascular dermal papillae projecting into the folded basal surface of the epidermis in the thick skin (see Micrograph #1 above). Sometimes due to plane of section a dermal papilla may appear as an isolated island of CT surrounded by epidermis. In addition to blood vessels, these folds on occasion contain sensory structures, Meissner's corpuscles. (Do not look for them in this slide. You will be asked to find one in a later slide. The deepest connective tissue of the dermis is coarser; the eccrine sweat glands continue into this layer and the subcutaneous (fatty) layer, but are better preserved on a later slide.

The dermis is divided into a papillary layer and a reticular layer. The papillary layer consists of the dermal papillae and immediately subjacent relatively loose connective tissue with blood vessels and relatively small collagen and elastic fibers (the collagen is more obvious in this stain). Note the blood vessels in some dermal papillae.

In thick skin, Meissner's corpuscles are present in other dermal papillae, but do not look for them in this slide. You will be asked to find one in a later slide. Eccrine sweat glands continue into this layer and the subcutaneous (fatty) layer, but are better preserved on a later slide.

The thin skin (axillary) on slide a-41 is less ridged. This section has a few hair follicles with associated sebaceous glands, as well as eccrine sweat glands. All of these epidermal appendages will be better seen in later slides. Identify the stratum granulosum in this thin skin section.

a-44 (slide# 2315). Fingertip. This preparation has less cell shrinkage than any of the other slides and is best suited for study of cellular detail. At high magnification you can see that the cytokeratin filaments are aggregated into tonofibrils that crisscross the cells in the stratum germinativum and spinosum and extend into cytoplasmic extensions where they anchor in the desmosomes. The eccrine sweat glands in this skin section are well developed, and their ducts (dark staining stratified cuboidal) can be distinguished from the lighter staining secretory portions.

Eccrine sweat glands are distributed over most of the body surface. They are simple coiled tubular glands; they consist of a highly coiled secretory portion deep in the dermis, and a relatively straight duct that conducts the secretions toward the surface and resorbs Na+ to render sweat hypotonic. The ducts enter the epidermis at the bases of the epidermal ridges. Within the epidermis the secretion is carried in corkscrew-shaped intercellular channels which open directly on the surface of the skin. In this case the ducts open in the center of ridges (cristae cutis) that form the fingerprint (dermatoglyph). Examine an eccrine sweat gland.

The secretory portion has both dark and light cells, with the dark cells having a few granules in the apices, and intercellular canaliculi can be seen between some of the light cells. Spindle shaped myoepithelial cells are present within the basement membrane (seen as elongated nuclei and/or pink cytoplasm).


Pacinian corpuscles, with their concentric layers of supportive cells around a central nerve fiber, can usually be seen on this slide in the deeper dermis (see Micrograph #2 right).

Meissner's corpuscles can be seen in some dermal papillae (see Micrograph #3 below. If you have difficulty in finding a Meissner's corpuscle, look at the Demonstration slide of this same section). Meissner's corpuscles are encapsulated tactile corpuscles and consist of flattened supportive cells arranged as horizontal lamellae surrounded by connective tissue capsule. Nerve fibers branch between the stacked lamellae. It is the horizontal lamellae that you will be able to find on your slide.

Skin color tones are mainly the result of varying amounts of the pigment melanin. Melanin is a product of specialized highly branched cells called melanocytes which are found in the epidermis. Melanin is also found in keratinocytes but the pigment is not formed in these cells; it is transferred to them from the melanocytes. In Caucasian skin (e.g. this slide), a few pigment granules can be seen in the cytoplasm of cells in the stratum basale, although it is difficult to tell which cells are melanocytes and which are keratinocytes with melanin accumulations. Merkel cell are also present in the stratum basale but cannot be distinguished from other cells without granules on this slide.

A melanocyte is not connected by desmosomes to other cells and therefore, after shrinkage, appears as a "clear" cell (i.e. surrounded by halo) in these sections. Some lab texts imply that all "clear" or lightly stained cells (with H&E stain) in the epidermis of lightly pigmented skin are melanocytes. This is misleading because Langerhans and Merkel cells are lightly stained by H&E in any epidermis and the Langerhans cells, like the melanocytes, tend to shrink away from the keratinocytes since they are not joined to them by desmosomes.

Melanosomes of keratinocytes are more numerous in races with highly pigmented skin (e.g. see Demonstration Slide H7480). In such skin, melanosomes can be found not only in the stratum basale but in the other epidermal layers as well.

See melanocyte in EM DEMO 22 FROM Lab #1. In this cell type, melanosomes are present but there are no large bundles of filaments.

See EM DEMO 3 of a Langerhans cell. This cell is important in the immune response since it is an antigen-presenting cell. Unusually shaped Birbeck granules (often tennis racket shaped) help to identify this cell type.

a-39 (93W7034) Axillary skin. Large apocrine sweat glands are found in certain regions of the body. They differ from the eccrine type in that the apocrine secretory portions are larger and the epithelium is usually a single type of cell. The term apocrine is probably inappropriate. In the past it was thought that the apical portion of the cell was actually pinched off, but this was probably due to fixation artifact. Myoepithelial cells are present within the basal laminae of eccrine and apocrine secretory glands. The nuclei of the myoepithelial cells can be distinguished most easily.

Ducts of both apocrine and eccrine sweat glands are very similar in appearance (with the former sometimes having a wider lumen). Apocrine and eccrine ducts consist of a mildly coiled tube composed of a stratified, low cuboidal epithelium (a double layer of cells, with those on the basal lamina relatively flat). The ducts also tend to stain more darkly, and lack myoepithelial cells. We do not ask you to establish whether the duct is apocrine or eccrine in origin. Since apocrine ducts are usually shorter than eccrine, most of the ducts you see will be eccrine.

a-45 (H10.51) External ear and a-40 (H7455) Scalp show haired skin. Slide a-45 is a somewhat thick section in which the sebaceous glands are well preserved. Note that the central cells are highly vacuolated (extracted lipid droplets) and some of the peripheral cells are undifferentiated. Find cross-cut and longitudinal sections of hair follicles on one of the two slides, preferably on a-40. In a longitudinal section you can see that the lower end of the follicle expands into a bulb into which is extended a connective tissue papilla containing blood vessels that ourish the follicle. Note that the sebaceous glands enter the pilosebaceous canal (i.e. space between hair shaft and follicle). Superficial to this entrance level the hair is free due to degeneration of the inner hair sheath; beneath this level the cuticle of the hair shaft interdigitates with that of the inner sheath. You are not required to learn the layers of the hair follicle.

Demonstration: Slide# H.7455. The arrector pili muscle (pointer) is a bundle of smooth muscle arising from the papillary layer of the dermis and attached to the connective tissue sheath of a hair follicle. When this muscle contracts, the hair is erected and the skin is depressed over the site of muscle attachment to the dermis (i.e. "goose-bump").

a-42 (H6270) shows elastic fibers of skin. The elastic fibers are stained black in this special preparation. The fibers in the papillary layer (outer surface of dermis in contact with epidermis) tend to be of smaller size than those deeper in the dermis reticular layer. There is no distinct boundary between these two layers of the dermis. If large blood vessels are present on your slide, you may also see elastic tissue in the walls of some of the vessels. You should be able to distinguish the reticular layer from the papillary layer on other skin slides you have studied.

Identification Practice

At this point in the course, you should now be able to understand much of what you see in any slide. Reexamine a-44 (2315). You should be able to find all of the described structures of the skin and, but also note nerves, vessels of various sizes, different types of connective tissue and muscle fibers. Try your hand as an histologist by identifying everything that you see on this slide.

Other Slides to Study
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Micrographs: